Forsythia Leaves Turning Yellow – Reasons For Yellow Leaves On Forsythia

Forsythis Leaves Turning Yellow
forsythia yellow leaves
(Image credit: Gardening Know How, via Nikki Tilley)

Forsythias are hardy, attractive bushes which delight us every spring with their early, golden blooms. The plants are relatively unbothered by insects and can withstand cold, heat and short periods of drought, but fungal diseases are a serious threat to their beauty. If you see your forsythia leaves turning yellow, it could be a sign of a significant fungal issue. Yellowing forsythia bushes are normal before fall leaf drop but during the growing season it’s time for action.

Why are Forsythia Leaves Turning Yellow?

Periods of rain during the warmer seasons create a humid, moist atmosphere which is perfect for the formation of fungus. Fungal spores can often live in soil for a long time and even overwinter there, bursting into infectious displays at the first sign of favorable weather. There are several fungal diseases that may be plaguing yellowing forsythia bushes. A fungicide can be helpful but it is best to try to identify the disease for more effective treatment. If you can rule out overcrowding, dry conditions and topical injury as well as any pests, you are left with a plant that probably has a fungal disease. Yellow leaves on forsythia occur from a variety of diseases, most of which can trace the vector to grafting or mechanical introduction, although disturbed soils may harbor spores for years. Maintaining a healthy plant through regular watering, fertilization, pruning and mulching can help minimize the damage from fungal disease. Forsythia leaf problems will not usually kill the plant, but due to its ornamental function, the disease can mar the beauty and dim the plant’s vigor.

Diagnosing Yellow Leaves on Forsythia

A forsythia with yellow leaves may be occurring due to any number of diseases. Below are the more common ones:

  • Yellow veins may indicate tobacco ringspot virus or arabis mosaic virus. Each is generally introduced through nematodes.
  • Yellow, black or brown spots that form a larger necrotic tissue can mean that forsythia with yellow leaves are caused by anthracnose, one of the most common fungal diseases on ornamental plants. The yellow tissue may also be colonized by tiny fruiting bodies.
  • Sclerotinia sclerotiorum begins with yellow leaves but advances to wilted stems and damage deepening to brown.

Treating Forsythia Leaf Problems

Fungicides are generally only effective if sprayed before the plant shows signs of disease. This is usually just at leaf formation. Once you notice a forsythia with yellow leaves, it is too late to make use of a fungicide. Treatment would include pruning to open the canopy and allow airflow through the plant and cleaning up any dead plant material around it. Minimize the splash of soil bound spores by watering gently at the base of the plant. Use a solution of 70 percent alcohol to clean any tools used to prune or rake around the plant. Maintain the plant’s vigor with regular watering, feeding and sterile pruning. The next year, in early spring, use a fungicide spray to prevent future occurrences. Forsythia leaf problems don’t announce the death knell of the plant but they are inconvenient and unsightly. Early prevention is key to preventing further issues.

Bonnie L. Grant

Bonnie Grant is a professional landscaper with a Certification in Urban Gardening. She has been gardening and writing for 15 years. A former professional chef, she has a passion for edible landscaping.